METHOD FOR MAKING A SPRING CORE FOR A MATTRESS OR FOR SEATING PRODUCTS

Abstract

A method for manufacturing a steel wire spring core for a mattress or seat is described, comprising the steps of providing a carrier made of steel wire; repeatedly cold-winding a steel wire spring from the steel wire taken from the carrier; and connecting a series of the coiled steel wire springs together. The steel wire has a diameter d between 0.8 and 4.5 mm and has a drawn pearlitic microstructure. The steel wire comprises a steel alloy having a carbon content between 0.35 wt% and 0.85 wt%. The steel wire in the carrier has a yield strength ratio (expressed as a percentage) of Rp0.2 (in MPa) to tensile strength Rm (in MPa) greater than 85%.

Description

METHOD FOR MAKING A SPRING CORE FOR A MATTRESS OR FOR SEATING PRODUCTS Description Technical Field The invention relates to methods for making a steel wire spring core for mattresses or seats. The steel wire spring core, for example, can be a pocket spring core, a Bonnell spring core, an LFK spring core, or a continuous wire spring core. Background Technique Different types of steel wire spring cores are known for use in mattresses or seating such as sofas. Examples of steel wire spring cores include pocket spring cores, Bonnell spring cores, LFK spring cores, and continuous wire spring cores. WO98 / 53933 describes a method and apparatus for forming a length of pocketed, connected coil springs for use in mattresses and the like. In the apparatus, steel wire from a supply is heated to between 232°C and 260°C by an induction heater, hot-coiled, cut, and cooled below a temperature where permanent deformation could occur from further processing of the spring. The spring is then compressed in preparation for insertion into a space provided by a fabric or stretchable material from a supply reel. The fabric is folded in on itself to provide the space. The temperature of the spring must also be sufficiently low for contact with the fabric without causing scorching or other damage. After the insertion of a compressed spring into the space, the fabric is ultrasonically welded to create individual but connected cavities for each spring.Next, the springs are oriented to allow each spring to expand, thus creating the length of the connected, pocketed coil springs. Document GB2347638A describes mattress spring units manufactured by forming a plurality of spring elements from a coil of steel wire. The rows of spring elements are secured together by lengths of helical wire until the desired size of spring unit is formed. The formed spring unit is then transferred to a furnace where it is tempered. After air cooling, the spring units are formed into either a coil or strips that are attached to the outer spring elements. During the tempering process, the overall height of the spring elements is reduced. Document W096 / 05109A1 describes a method for producing bagged coil springs for use in internal spring constructions. The method comprises the steps of forming coil springs from spring wire at a first temperature—where the spring wire has inherent residual stresses; conditioning the coil springs at a second temperature sufficient to substantially reduce the inherent residual stresses in the spring wire of the coil springs; adjusting the temperature of the conditioned coil springs to a third temperature sufficient to permit the insertion of the conditioned coil springs into a fabric bag; and inserting the coil springs into a fabric bag. Description of the Invention The invention is a method for manufacturing a steel wire spring core for a mattress or seat. The method comprises the steps of providing a carrier comprising steel wire; repeatedly cold-winding a steel wire spring from the steel wire taken from the carrier; and connecting a series of the wound steel wire springs together. Preferably, the steel wire spring is a helically wound steel wire spring. The steel wire has a diameter d between 0.5 and 4.5 mm. The steel wire comprises a steel alloy having a carbon content between 0.35 wt% and 0.85 wt%. The steel wire has a drawn pearlitic microstructure. The steel wire in the carrier has a yield strength ratio (RPo), expressed as a percentage.2 (in MPa) over the tensile strength Rm (in MPa) higher than 85%, preferably higher than 87%, even more preferably higher than 90%, even more preferably higher than 92%, and still more preferably higher than 93%. The mechanical properties Rm and Rpo.2 are defined and tested in accordance with ISO 6892-1:2016. The tensile strength Rpo.2 is the maximum stress (in MPa) in the tensile test. The yield strength Rpo.2 (in MPa) is the stress when the stress crosses the line through the 0.2% strain and parallel to the elastic modulus line. The ratio Rpo.2 / Rm is the value of Rpo.2 (in MPa) divided by the value for Rm (in MPa) and expressed as a percentage. Cold winding means that the winding is done at room temperature, meaning that the wire is not heated to wind the steel wire spring. The use of specific steel wire properties, compared to the steel wire in the carrier, to wind steel wire springs ensures that a steel wire spring is manufactured at high speeds in a reliable and consistent manner, exhibiting little to no relaxation when used in a steel wire spring core in a mattress or seating product. As a result, local permanent deformation of the wire core will be minimal when used in mattresses or seating products. The use of the carrier comprising the steel wire, as in the invention, eliminates the need for special heat treatments on the spring winding machine before or after spring winding, or on the steel wire spring core, to reduce local permanent deformation of the steel wire spring cores when used in mattresses or seating products. (Special) Heat treatments on the steel wire on the spring winding machine, or on the steel spring wound on the spring winding machine, cannot be performed reliably and consistently, also due to the increased speeds of spring winding. Using cold coiling to make springs allows them to be coiled at high speeds, as the steel wire does not need to be heated in the spring coiling machine. The textile fabric – typically a non-interlaced polymer fiber fabric – of pocketed spring cores is not sufficiently temperature resistant to withstand subsequent heat treatment in pocketed spring cores to reduce or eliminate relaxation of the steel wire springs of pocketed spring cores. The steel wire used in the method of the invention can be produced by drawing steel wire starting from a steel wire rod. Drawn steel wires having a drawn pearlite microstructure typically have an Rpo-2 value of approximately 70-75% of the tensile strength Rm. By heat-treating the steel wire at temperatures between 200 and 300°C, the Rpo-2 value relative to the tensile strength Rm is increased to the levels specified in the invention. The heat treatment can be performed as an in-line process at the end of the wire drawing, or offline in a batch process in a furnace. Preferably, the steel alloy comprises more than 0.55 wt% C, and even more preferably more than 0.6 wt% C. Still more preferably, the steel alloy comprises more than 0.7 wt% C. Such embodiments are particularly preferred. The higher carbon content of the steel alloy provides steel wires with higher strength (higher Rm values). The relatively high RPo.2 values ​​of the steel wires used in the invention mean that the absolute RPo.2 value is even higher in such embodiments. This is favorable to the invention as it provides mattress spring cores with even less spring relaxation. Ideally, the elongation at break in the tensile test of the steel wire is greater than 3%. Preferably, the steel alloy comprises between 0.1 and 1.4% by weight of Si; and preferably less than 0.8% by weight of Si; more preferably less than 0.3% by weight of Si. Optionally, the steel alloy may comprise micro-alloying elements in individual quantities less than 0.5% by weight; even more preferably in individual quantities less than 0.3% by weight. Examples of such micro-alloying elements are Cr, W, V, Mo, Ti, and Nb. The steel alloy also comprises unavoidable impurities: preferably phosphorus is limited to 0.035% by weight, preferably sulfur is limited to less than 0.035% by weight, preferably aluminum is limited to less than 0.1% by weight; and preferably copper is limited to less than 0.2% by weight. In preferred embodiments, the steel alloy does not comprise—beyond impurity levels—any of the following microalloying elements: Cr, W, V, Mo, Ti, Nb. The steel wire further comprises unavoidable impurities: preferably, phosphorus is limited to 0.035 wt%, sulfur is limited to less than 0.035 wt%, aluminum is limited to less than 0.1 wt%, and copper is limited to less than 0.2 wt%. In a more preferred embodiment, the steel alloy comprises Mn and Si, with the remainder of the steel alloy composition being iron. Ideally, the steel wire has a diameter that varies between 1.6 mm and 2.5 mm. Ideally, the steel wire has a diameter greater than 1.7 mm. Ideally, the steel wire has a diameter less than 2.3 mm. Ideally, the steel wire has a diameter between 1.7 mm and 2.3 mm. In a preferred method, the steel alloy comprises between 0.2 and 0.9 wt% of Mn; more preferably more than 0.4 wt% of Mn. Preferably, the steel alloy comprises between 0.3 and 1.6 wt% of Si and between 0.6 and 0.9 wt% of Cr. More preferably, the steel alloy consists of between 0.35 and 0.85 wt% of C, between 1.3 and 1.6 wt% of Si, between 0.6 and 0.9 wt% of Cr, unavoidable impurities, and the remainder being iron. In a preferred method, the carrier is a coil around which the steel wire is wound. This method is preferred because the use of other carriers could negatively affect the mechanical properties of the steel wire within the carrier. For example, the use of a spider is less preferred because the steel wire needs to be deformed to fit it into the spider, and this deformation can negatively affect the mechanical properties of the steel wire relevant to compression springs. In a preferred method, the steel alloy of the steel wire comprises between 0.02 and 0.06% aluminum by weight. This method is preferred because the spring coiling is improved due to the presence of aluminum in the steel alloy, which enhances the ductility of the steel wire. In a preferred method, more than 120 steel wire springs are manufactured per minute. Preferably, the tensile strength Rm (in MPa) of the steel wire is higher than the value obtained using the formula 2200 - 390.71*In(d); where d is the diameter of the steel wire in mm, and In(d) is the natural logarithm of the diameter d in mm. More preferably, the diameter of the steel wire in such configurations is less than 1.7 mm; even more preferably, less than 1.6 mm. For clarity, an example calculation is provided: for a steel wire with a diameter of 1.5 mm, the formula 2200 - 390.72*In (1.5) yields 2041.6 MPa. Preferably, the tensile strength Rm (in MPa) of the steel wire is less than the value obtained by means of the formula 2450 - 390.71*In(d); where d is the diameter of the steel wire in mm. In a preferred method, the steel wire does not comprise a metallic coating. In the case of no metallic coating, the steel wire is preferably provided with an oil or wax for protection against corrosion. In an alternative preferred method, the steel wire is provided with a metallic coating. Preferably, the metallic coating comprises or consists of zinc; or comprises at least 84% by weight of zinc and optionally aluminum. More preferably, the microstructure of the metallic coating comprises a globular aluminum-rich phase. Such a globular aluminum-rich phase is created particularly when heat treatment is performed on the steel wire, whether the heat treatment is carried out in-line (meaning in a continuous operation) or in a batch process. It is believed that the globular aluminum-rich phase improves the corrosion resistance of the metallic coating layer, such that a thinner metallic coating layer can be used while still providing corrosion protection. Preferably when the steel wire comprises a metallic coating, the amount of metallic coating is less than 120 g / m2, more preferably less than 80 g / m2, even more preferably less than 60 g / m2. Viewed from an alternative and general aspect, the invention is directed at saving weight in steel wire spring cores for mattress or seat that still exhibit low relaxation properties. The following formula determines the spring rate R of a steel spring: R = GX d4 / (8Na x Dm3) where d is the wire diameter, Dm is the spring diameter, Na is the number of coils, and G is the shear modulus. Thus, the stiffness of a spring is proportional to the fourth power of the wire diameter and inversely proportional to the number of coils. To save weight, the wire diameter can be reduced. In order to maintain the spring rate (R) at the same level for a spring with the same diameter and height, the number of coils (Na) must be decreased. The reduced number of coils (Na) leads to a shorter length of steel wire in the spring. Thus, the weight-saving effect is twofold: a thinner diameter steel wire and a shorter length of steel wire. For the same amount of spring compression, however, the steel wire is subjected to a higher degree of twist due to the reduced number of coils (Na). Because of this higher degree of twist, the steel wire is at risk of yielding more quickly in the plastic region. Therefore, the steel wires must exhibit higher elastic strength to prevent plastic deformation and ensure the steel wires can rebound multiple times. The elastic resistance Rpo.2 of steel wires expressed in MPa is preferably higher than the value obtained by the formula 1870 - 332.10 x In (d) , and much more preferably higher than the value obtained by the formula 1980 - 351.63 x In (d), where d is the diameter of the wire expressed in mm. POCKET SPRING CORES MADE OF LINEAR STRANDS OF POCKET SPRINGS In a preferred method, a series of coiled or spiral steel wire springs are connected by inserting the coiled steel wire springs in a compressed state into pockets made of fabric. A linear chain of pocket springs is thus obtained. Pocket spring cores are made using this method. More preferably, the pockets of the linear chain of pocket springs are formed from a single piece of fabric. Even more preferably, the pockets are closed, resulting in a linear chain of pocket springs. A spring core unit for a mattress can be made by connecting (preferably by gluing) parallel linear chains of pocket springs together.It is a benefit of this preferred method—where the connection of a series of wound steel wire springs is made by inserting the wound steel wire springs in a compressed state into pockets made of fabric—that lighter steel wire spring cores can be made. In the prior art, the springs are compressed and placed in the pockets. The free height of the springs (that is, the height when no compressive load is exerted on the springs) of the pocketed spring cores of the prior art is greater than the height of the pocket. The resulting pre-compression of the springs in the pockets helps prevent the occurrence of permanent compression deformation of mattresses made with the spring cores.Since in the invention the springs are less prone to permanent compression deformation, less pre-compression of the springs in the pockets is required, and in this way the free height of the spring can be less, resulting in a lighter pocketed spring core. POCKET-WRAPPED SPRING CORES MADE DIRECTLY WITH A TWO-DIMENSIONAL ARRAY OF POCKET-WRAPPED SPRINGS - more specifically so-called micro-coil spring cores or nano-coil spring cores In a preferred method, a two-dimensional array of coiled or spiral steel wire springs is provided. The coiled steel wire springs are enclosed in bags. The plane of the two-dimensional array is perpendicular to the longitudinal axes of the coiled steel wire springs. The bags are formed by a first layer of fabric on top of the coiled steel wire springs, a second layer of fabric below the coiled steel wire springs, and seams between the first and second layers of fabric. The seams encircle the coiled steel wire spring. Preferably, the first and second layers of fabric are woven from thermoplastic fibers; more preferably, non-woven thermoplastic fibers, for example, yarn-bonded non-woven fabrics.Ideally, the welds are welded seams, which thermally bond the first layer of thermoplastic fabric to the second layer of thermoplastic fabric. Preferably, such a steel wire spring core has a height less than 6 cm, more preferably less than 5 cm, and even more preferably less than 4 cm. In this preferred method, the diameter of the steel wire is preferably less than 1 mm, for example 0.8 mm. Such methods are of particular interest when making low-height steel wire spring cores (e.g., less than 5 cm high). In this preferred method, a low-height spring core can be made with little or no relaxation. In a more preferred method, more than 200 springs are manufactured per minute. An advantage of these preferred methods is that low-profile spring cores can be manufactured for use as a comfort layer in a mattress, placed on top of another spring core, such as a pocket spring core. Such a comfort layer, made according to the invention, is breathable and elastic in multiple directions, with limited or no relaxation. It is proposed that limited or permanent deformation of the springs will occur when the spring core is used. The high manufacturing speeds of steel wire springs and the specific fabric selection (mostly non-interlaced polymer fiber fabrics) make it virtually impossible to perform heating operations on the steel wire or the wound steel wire springs in the spring manufacturing machine and / or the spring core manufacturing machine. In one version, the first layer of fabric and the second layer of fabric can be two different fabrics. In another configuration, the first and second layers of fabric can be a single layer of fabric folded over the other. BONNELL OR LFK TYPE SPRING CORES A preferred method involves connecting the coiled or spiral springs together by tying a steel wire through the coiled springs. More preferably, the springs are individually coiled and supplied as discrete parts to the operation, where the steel wire is tied through the coiled springs to interconnect them. In this way, Bonell-type or LFK-type spring cores can be produced. In a preferred embodiment, the coil springs have a knot at both ends, provided by the steel wire from which the springs are wound, which knots the steel wire to itself within the spring. More preferably, a steel wire is tied through the coil springs to connect them together. In this way, a Bonell-type wire core can be made. In a preferred embodiment, the coil springs do not have a knot at either end provided by the steel wire from which the springs are wound. More preferably, a steel wire is tied through the coil springs to connect them together. In this way, LFK-type spring cores can be made. CONTINUOUS WIRE SPRING CORES In a preferred embodiment, a multitude of steel wire coils are wound without cutting the steel wire, such that the steel wire runs continuously through the multitude of steel wire coils in the spring core. In this way, a continuous coil spring core is manufactured for a mattress or seat. Optionally, an additional tie wire may be used to improve the interconnection between the steel wire coils. Brief Description of the Figures in the Drawings Figure 1 illustrates the tensile stress-strain curve of a steel wire. Figure 2 shows a pocket spring mattress core as can be made using the method of the invention. Figure 3 shows an example of a Bonnell spring. Figure 4 shows a Bonnell spring core for a mattress, as can be made using the method of the invention. Figure 5 shows an example of an LFK spring. Figure 6 shows an LFK spring core for a mattress, as can be made using the method of the invention. Figure 7 shows a continuous spring, as can be made using the method of the invention. Figure 8 shows another type of steel wire spring core where the steel wire spring is enclosed in fabric bags. Mode(s) for Carrying Out the Invention Figure 1 provides information about the mechanical properties of the steel wires described in this document. The mechanical properties are described and tested according to ISO 6892-1:2016 (titled Metallic materials - Tensile testing - Part 1: Test method at room temperature). Figure 1 schematically illustrates a stress-strain curve of a steel wire in a uniaxial tensile test. The X-axis represents strain. The vertical (Y) axis represents tensile strength (in MPa). Elongation at break is represented by Δt. The tensile strength R is the ultimate tensile stress. The yield strength R0.2 is the stress at which the tensile curve intersects the line through 0.2% strain and is parallel to the elastic modulus line. Figure 2 shows a pocket spring mattress core as made using the method of the invention. Figure 3 shows an example of a Bonnell spring. Figure 4 shows a Bonnell spring core for a mattress, as made using the method of the invention. Figure 5 shows an LFK spring. Figure 6 shows an LFK spring core for a mattress, as made using the method of the invention. Figure 7 shows a continuous spring as made for manufacturing a mattress core using the method of the invention. Figure 8 shows another type of steel wire spring core where the steel wire springs are enclosed in fabric, and which can be made by a method according to the invention. The steel wire springs are positioned in a two-dimensional array. A non-woven fabric is provided above and below the two-dimensional arrangement of the steel wire springs. The pockets are formed by a first non-woven fabric above the coiled steel wire springs, by a second non-woven fabric below the coiled steel wire springs, and by seams between the first and second non-woven fabrics. The seams encircle the coiled steel wire springs. The seams can be established by thermal welding (e.g., by means of ultrasonic welding equipment) that joins the two non-woven fabrics together. A first series of experiments relates to pocket spring cores for mattresses. A 2 mm diameter steel wire was used, made from a steel alloy consisting of between 0.71 and 0.75% by weight of carbon, between 0.6 and 0.9% by weight of manganese, at most 0.03% by weight of aluminum (unavoidable impurities), and the remainder being iron. A 2 mm diameter steel wire was drawn from a 5.5 mm diameter wire rod. The tensile properties of the steel wire were tested according to ISO 6892-1:2016: tensile strength Rm = 2018 MPa, RPo.2 = 1507 MPa (meaning that RPo.2 is 75% of the tensile strength Rm), and elongation at break = 3%. A coil of steel wire was treated in an oven at 300°C for 2 hours. After this heat treatment, the tensile properties were tested again: tensile strength Rm = 2052 MPa, RPo. 2 = 1823 MPa (which means that the RPo.2 is 89% of the tensile strength Rm), elongation at break 7%. The helically coiled springs were manufactured according to the pocket spring design using oven-treated steel wire, as described in the previous paragraph. The spring height was 210 mm, the spring diameter was 80 mm, and each spring had 7 coils. The springs were tested according to the Brazilian standard ABNT 15413-1:2013, entitled "Spring Mattresses and Bases - Part 1: Requirements and Test Methods." This part of ABNT NBR 15413 establishes the requirements and test methods for spring mattresses and bases. The test method described in this standard involves manually compressing a single spring to full compression for 10 seconds. After removing the load and allowing the spring to recover, another cycle of manual compression to full compression is performed for 10 seconds.After removing the load and allowing the spring to recover, a new manual compression cycle to full compression is performed for 60 seconds. After load removal, the spring's height loss compared to its initial height is measured and expressed as a percentage of the initial spring height. A maximum height loss of 8% is acceptable according to this Brazilian standard. This test, performed on helically coiled springs with heat-treated steel wire, showed no height loss. The fatigue strength of spring cores made according to the method of the invention has been tested: the effect is that helically wound springs made of steel wire that have been subjected to heat treatment are highly resistant to fatigue strength. A second series of tests relates to steel wires for making Bonnell spring cores. A 2.2 mm diameter steel wire was made from a steel alloy consisting of between 0.55 and 0.59 wt% carbon and between 0.6 and 0.9 wt% manganese, with a maximum of 0.03 wt% aluminum; unavoidable impurities, the remainder being iron. The steel wire was drawn to the 2.2 mm diameter starting from a 5.5 mm diameter wire rod. The tensile properties of the steel wire were tested according to ISO. 6892-1:2016: tensile strength Rm = 1415 MPa, RPo.2 = 1050 MPa (meaning that RPo.2 is 74.2% of the tensile strength Rm), and elongation at break = 3.25%. The steel wire coils were heat-treated in a furnace at different temperatures for one hour. After this heat treatment, the tensile properties were tested again, and the results are given in Table I. The first column of Table I indicates the temperature at which the heat treatment was performed in the furnace. Temperature (°C) Rm (MPa) Rpo.2 (MPa) RP0.2 / Rm (%) Elongation (%) 200 1507 1427 94.7 1.5 220 1526 1453 95.2 2 240 1547 1465 94.7 2 260 1557 1449 93.1 3.5 280 1525 1420 93.2 2.5 300 1517 1427 94.0 5 Table I - Results of the tensile test of the heat-treated coil steel wire The table below illustrates how the invention can be applied to achieve weight savings in steel wire spring cores for mattresses or seats. Wire diameter (mm) D 2 1.8 Tensile strength Rm 1800 1800 Minimum (MPa) Spring outer diameter (mm) Douter 65 65 Free spring length (mm) Llibre 160 160 Solid height (mm) Lsolida 14 9 Max working length (mm) Ln 35.9 31.7 Number of active coils Na 6 4 Spring index c 31.5 35.1 Wahl correction factor W 1.0 1.0 Coil spacing (mm) Pitsch 26.3 39.6 Lift angle (°) θ 7.6 11.3 Spring rate (N / mm) R 0.106 0.103 Max load at solid height (N) Fmax 15.5 15.6 Max total shear stress (MPa) Tmax 324.4 448.2 Max working load (N) Fmax load 13.2 13.3 Max working shear stress (MPa) Tmax load 275.8 380.9 Allowable shear stress Max (MPa) T zul 1008 1008 Safety margin in shear stress 3.11 2.25 Maximum possible displacement (mm) Lalambre 146 151 Length required to make the spring (mm) Lalambre 1.6 1.2 Mass of the spring (kg) Mresorte 0.039 0.024 Table II Following the results of Table II, weight savings of 39% are achieved.

Claims

1. A method for manufacturing a steel wire spring core for a mattress or seat, characterized in that it comprises the steps of: providing a carrier comprising steel wire; repeatedly cold-winding a steel wire spring from the steel wire taken from the carrier; and connecting a series of the coiled steel wire springs together; wherein the steel wire has a diameter d between 0.5 and 4.5 mm; wherein the steel wire comprises a steel alloy, wherein the steel alloy has a carbon content between 0.35 wt% and 0.85 wt%; wherein the steel wire has a drawn pearlitic microstructure; wherein the steel wire in the carrier has a ratio—expressed as a percentage—of the elastic strength RPo.2 (in MPa) to the tensile strength Rm (in MPa) higher than 85%.

2. The method according to claim 1, characterized in that the steel alloy has a carbon content higher than 0.6% by weight, preferably higher than 0.7% by weight.

3. The method in accordance with any of the preceding claims, characterized in that the carrier is a coil on which the steel wire is wound.

4. The method according to any of the preceding claims, characterized in that the steel alloy comprises between 1.3 and 1.6% by weight of Si; and between 0.6 and 0.9% by weight of Cr.

5. The method according to claim 4, characterized in that the steel alloy consists of between 0.35 and 0.85% by weight of C, between 1.3 and 1.6% by weight of Si, between 0.6 and 0.9% by weight of Cr, unavoidable impurities and the remainder being iron.

6. The method in accordance with any of the preceding claims, characterized in that the steel alloy comprises between 0.02 and 0.06% by weight of aluminum.

7. The method in accordance with any of the preceding claims, characterized in that more than 120 steel wire springs are manufactured per minute.

8. The method in accordance with any of the preceding claims, characterized in that the tensile strength Rm (in MPa) of the steel wire is higher than the value obtained by means of the formula 2200 390.71* In (d); where d is the diameter of the steel wire in mm.

9. The method according to any of claims 1-8, characterized in that the steel wire does not comprise a metallic coating layer.

10. The method according to any of claims 1-8, characterized in that the steel wire is provided with a metallic coating, preferably wherein the microstructure of the metallic coating comprises a globular aluminum-rich phase.

11. The method according to any of the preceding claims 1-10, characterized in that the connection of a series of spiral steel wire springs to each other is made by inserting the spiral steel wire springs in a compressed state into bags made of a fabric, where a linear chain of bagged springs is obtained.

12. The method according to claim 11, characterized in that the bags are formed from a single piece of fabric and wherein the bags are closed and linearly joined together by means of welded joints.

13. The method according to any of the preceding claims 1-10, characterized in that a two-dimensional array of spiral steel wire springs is provided, wherein the plane of the two-dimensional array is perpendicular to the longitudinal axes of the spiral steel wire springs; wherein the spiral steel wire springs are enclosed in bags; wherein the bags are formed from a first layer of fabric on top of the spiral steel wire springs, by a second layer of fabric below the spiral steel wire springs, and by seams between the first layer of fabric and the second layer of fabric, wherein the seams encircle the spiral steel wire springs.

14. The method according to any of the preceding claims 1-10, characterized in that it comprises the step of connecting the spiral springs together by tying a steel wire through the spiral springs.

15. The method according to claims 1-10, characterized in that a multitude of steel wire springs are wound without cutting the steel wire such that the steel wire runs continuously through the multitude of steel wire springs in the spring core.